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TSS721A: mbus slave rx ONLY

Daniel Krüger
Prodigy 10 points

Part Number: TSS721A

Hello!

We try to designe a m-bus slave around TSS721A. RX from mbus master read juste fine, and TX answer echo from tx to rx the same bits indicating optocouples do work. (MCU-TSS721A = OK)

Problem is that TSS721A chip does not modulate current signal on mbus and Master does not receive any data. Anyone have a clue? Tech pdf from TI is studied but to no luck.

  • 0
    TI__Expert 3530 points

    Hi Daniel,

    Could you send a schematic? My first guess would be something up with the RIS resistor.

  • 0 in reply to Ethan White
    Prodigy 10 points

    yes, I agree, but I think RIS is OK. Is it OK to use RX and not RXI? as long as echo is same as tx, this should be OK?

  • 0 in reply to Daniel Krüger
    TI__Expert 3530 points

    Hi Daniel,

    I'd try increasing R_RIDD to 30kOhms. That's typically what I see in our reference designs for slave modules. My guess is that the lower resistance is causing some higher currents somewhere causing issues with the capacitor charging/discharging. 

  • 0 in reply to Ethan White
    Prodigy 10 points

    thankyou! I have tried this, but then MCU dont get any data from TSS/mbus master. This may indicate my optocoupler draw too much current when its fed by the tss.  with 2UL mcu read OK, send ACK back to tss721, and read echo in return, but TSS does not mudulate current on Mbus. I will try to laborate with resistors and caps. per your suspision it must be some strange current somewhere that causes this. wery little info on the internet even TSS chip has existed forever:-)

  • 0 in reply to Daniel Krüger
    TI__Expert 3530 points

    Hi Daniel,

    One thing you can try is to probe VDD when you try to transmit. You might see the voltage dipping significantly, indicating too much current being pulled from the C_STC capacitor which would deactivate the slave supply. In that case you could try to significantly increase C_STC (like 2x or 3x what it currently is). Or you could try to use a lower power optocoupler, though I'm not sure what kind of current yours draws, the only datasheet I could find wasn't in English :)

    -Ethan White